Would a Green Line branch to Allston make more sense? Just continue onto Brighton Ave from Commonwealth B line. A separate Blue extension that way seems redundant.
Blue running down Brookline Ave would be nice to give actual Fenway the Fenway stop (it's currently weirdly out of the way and not central to all the density being built down Brookline and Boylston).
Is there a reason for doubling up lines on top of each other other than expediency of the below grade one? Or are you suggesting just replacing the Green E entirely with Blue subway?
Replying here. "Blue-to-Allston" isn't so much about bringing the Blue Line to Allston specifically but for opening it up to continue on to places like Harvard, Newton Corner, Watertown, Waltham, and Riverside. Green Line branch(es) itself are almost certainly the better play for service to Allston itself, but if you wanted to go farther than Newtown Corner, I think it becomes less appealing, which is why it would be a question of whether Blue is better (especially if Blue runs largely express through Allston via a Mass Pike alignment). Hardly a sure thing, but that's why it's typically separated from the more modest Blue-to-Kenmore extension, especially since...
...Blue extensions to the south of Kenmore are also a mixed bag. Believe me, you don't need to sell me on the raw benefits of a subway down Brookline Ave -- the tragedy of transit access to LMA is a repeated saga of "close but not quite there". But I can't even see a subway being built there, if for no other reason than you have a perfectly good ROW just to the west. Yeah, the walking catchments overlap poorly because of the park in between, but I can't imagine the benefits outweighing the costs.
I think it'd be a better choice to put bus lanes and related infrastructure down Brookline Ave and Longwood Ave and do our best to make that be BRT-esque.
Blue to Brookline Village via the existing LRT ROW isn't a
bad idea, but it has its own problems, chief among them being that you can't go any further out on the Highland Branch: any future extension to Needham is going to involve several grade crossings in Downtown Needham, and those will require LRT rather than HRT. If you run Blue past Brookline Village, you either block or hamstring severely an extension to Needham (which is badly needed in order to free up space on the Northeast Corridor).
I don't think I was clear earlier -- definitely not proposing replacing the Green E with the Blue Subway. Rather, I'm proposing that Surface Green E be replaced with an extended Subway Green E + D: extend the Huntington Subway down to Riverway, and then connect it to the D Line at Brookline Village, and extend the E to Needham.
In general, I agree, it's better to avoid doubling up lines. However, it's worth saying that the Green Line makes for a theoretical exception to that rule in some cases, because a surface level Green Line could act as a "local" service while a subway underneath could run as "express." The Green Line is always a little bit of an exception to every rule because it can exist on a sliding scale where you have HRT-style rapid transit on one end and local bus on the other.
Case in point -- if the Blue
did get extended to Allston, I would imagine that it
doesn't replace any particular Green Line branch, but rather operates as an express bypass.